In 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP-LTE (hereinafter referred to as LTE)), Orthogonal Frequency Division Multiple Access (OFDMA) is adopted as a downlink communication scheme, and Single Carrier Frequency Division Multiple Access (SC-FDMA) is adopted as an uplink communication scheme (e.g., see NPL-1, NPL-2, and NPL-3).
In LTE, a radio communication base station apparatus (hereinafter abbreviated as “base station”) performs communication by allocating a resource block (RB) in a system band to a radio communication terminal apparatus (hereinafter abbreviated as “terminal”) for every time unit called “subframe.” The base station also transmits assignment control information (i.e., L1/L2 control information) for indicating the result of resource allocation for downlink data and uplink data to the terminal. The assignment control information is transmitted to the terminal through a downlink control channel such as a Physical Downlink Control Channel (PDCCH).
Here, a resource region to which a PDCCH is to be mapped is specified, and as shown in FIG. 1, a PDCCH covers the entire system bandwidth in the frequency-axis direction and the region occupied by the PDCCH in the time-axis direction varies between the top OFDM symbol and third OFDM symbol in a single subframe. The signal indicating the range of OFDM symbols occupied by a PDCCH in the time direction is indicated by the base station to the terminal through a Physical Control Format Indicator Channel (PCFICH).
Each PDCCH also occupies a resource consisting of one or more consecutive control channel elements (CCEs). In a PDCCH, one CCE consists of 36 resource elements (RE). In LTE, the number of CCEs occupied by a PDCCH (CCE aggregation level, or simply aggregation level) is selected from 1, 2, 4, and 8 depending on the number of information bits of assignment control information or the channel condition of a terminal. In LTE, a frequency band having a system bandwidth of up to 20 MHz is supported.
Assignment control information transmitted from a base station is referred to as downlink control information (DCI). When assigning a plurality of terminals to one subframe, the base station transmits a plurality of DCIs, simultaneously. In this case, in order to identify the terminal of the transmission destination of each DCI, the base station transmits the DCI with CRC bits masked (or scrambled) with the terminal ID of the transmission destination and included in the DCI. Then, the terminal performs de-masking (or descrambling) of the CRC bits of a plurality of DCIs possibly directed to the terminal, with its own ID, thereby blind-decoding a PDCCH to detect the DCI directed to the terminal.
The DCI also includes information (resource allocation information (for example, the number of allocated resource blocks)) regarding a Physical Downlink Shared Channel (PDSCH) resource and a Physical Uplink Shared Channel (PUSCH) resource allocated to a terminal by the base station. In addition, DCI includes a Modulation and channel Coding Scheme (MCS) (for example, information indicating an M-ary modulation number and/or a transport block size with respect to the number of allocated resource blocks) allocated to the terminal by the base station, for example. Furthermore, the DCI has a plurality of formats for uplink, downlink Multiple Input Multiple Output (MIMO) transmission, and downlink non-consecutive band allocation. The terminal needs to receive both downlink assignment control information (i.e., assignment control information about downlink: DL assignment) and uplink assignment control information (i.e., assignment control information about uplink: UL grant) which have a plurality of formats.
For downlink assignment control information, formats of a plurality of sizes are defined in accordance with a transmission antenna control method or a resource assignment method for a base station, for example. Among the plurality of formats, a downlink assignment control information format for consecutive band allocation (hereinafter simply referred to as “downlink assignment control information”) and an uplink assignment control information format for consecutive band allocation (hereinafter simply referred to as “uplink assignment control information”) have the same size. These formats (i.e., DCI formats) include type information (for example, a one-bit flag) indicating the type of assignment control information (downlink assignment control information or uplink assignment control information). Thus, even if the DCI indicating downlink assignment control information and DCI indicating uplink assignment control information have the same size, the terminal can determine whether the specific DCI indicates downlink assignment control information or uplink assignment control information by checking the type information included in assignment control information.
The DCI format in which uplink assignment control information for consecutive band allocation is transmitted is referred to as “DCI format 0” (hereinafter referred to as “DCI 0”), and the DCI format in which downlink assignment control information for consecutive band allocation is transmitted is referred to as “DCI format 1A” (hereinafter referred to as “DCI 1A”). DCI 0 and DCI 1A are of the same size and distinguishable from each other by referring to the type information as described above. For this reason, DCI 0 and DCI 1A will be collectively referred to as DCI 0/1A, hereinafter.
In addition to these DCI formats, there are other formats for downlink, such as DCI format 1 used for non-consecutive band allocation (hereinafter referred to as DCI 1) and DCI formats 2 and 2A used for assigning spatial multiplexing MIMO transmission (hereinafter referred to as DCIs 2 and 2A, respectively).
DCI 1, DCI 2, and DCI 2A are formats that are used dependently on the downlink transmission mode of a terminal (non-consecutive band allocation or spatial multiplexing MIMO transmission) and are configured for each terminal. In contrast, DCI 0/1A is a format that is used independently of the transmission mode and is usable for a terminal in any transmission mode, i.e., a format commonly used for every terminal. When DCI 0/1A is used, single-antenna transmission or a transmit diversity scheme is used as the default transmission mode.
Also, the standardization of 3GPP LTE-Advanced (hereinafter referred to as LTE-A), which provides a data transfer rate higher than that of LTE, has been started while keeping backward compatibility with LTE. In LTE-A, in order to achieve a downlink transfer rate of up to 1 Gbps or above and an uplink transfer rate of up to 500 Mbps or above, it is expected that base stations capable of performing communication using a wideband frequency of 40 MHz or higher and terminals designed for an optional LTE-A system (hereinafter referred to as LTE-A terminals) will be introduced. In addition, the LTE-A system is required to provide coverage supporting not only LTE-A terminals but also terminals designed for an LTE system (hereinafter referred to as LTE terminals).
In order to simultaneously achieve communication at an ultra-high speed transmission rate several times faster than a transmission rate in the LTE system and backward compatibility with the LTE system, the band for the LTE-A system is divided into “component bands” of 20 MHz or less, which is a bandwidth supported by the LTE system. That is, the “component band” is a band having a bandwidth of a maximum of 20 MHz and is defined as a base unit for the communication band.
In a Frequency Division Duplex (FDD) system, a “component band” in downlink (hereinafter, referred to as “downlink component band”) may further be defined as a band resulting from division by downlink frequency band information in a BCH broadcast from a base station or a band defined by a distributed bandwidth when a downlink control channel (PDCCH) is distributed over the frequency domain. A “component band” in uplink (hereinafter, referred to as “uplink component band”) may be defined as a base unit of a communication band of 20 MHz or less including a band resulting from division by uplink frequency band information in a BCH broadcast from a base station or a Physical Uplink Shared Channel (PUSCH) region near the center and including an LTE compatible PUCCH at both ends. Note that in 3GPP LTE-Advanced, a “component band” may be expressed as component carrier(s) or cell in English. In addition, the component carrier(s) may be abbreviated as CC(s).
The LTE-A system supports communication using a band which bundles several component carriers, so-called carrier aggregation. The throughput requirement for uplink communication is generally different from the throughput requirement for downlink communication, and therefore, studies are carried out on carrier aggregation in which the number of uplink component carriers and the number of downlink component carriers configured for LTE-A terminals are different, so-called asymmetric carrier aggregation in the LTE-A system. Furthermore, the LTE-A system supports a case where the number of uplink component carriers is asymmetric to the number of downlink component carriers and the respective component carriers have different frequency bandwidths.
In a Time Division Duplex (TDD) system, the downlink component carrier and uplink component carrier have the same frequency band, and downlink and uplink are switched on a time-division basis to achieve downlink communication and uplink communication. Therefore, in the case of TDD system, the downlink component carrier can be expressed as “downlink communication timing in the component carrier.” Meanwhile, the uplink component carrier can be expressed as “uplink communication timing in the component carrier.”
The downlink component carrier and uplink component carrier are switched based on a frame format called “UL-DL configuration” as shown in FIG. 2. The UL-DL configuration is indicated to a terminal via a broadcast signal called System Information Block (SIB) when the terminal starts communication with the base station. In the UL-DL configuration shown in FIG. 2, timings in subframe units (that is, 1 msec unit) of downlink communication (DL: Downlink) and uplink communication (UL: Uplink) per frame (10 msec) are configured. The UL-DL configuration makes it possible to construct a communication system capable of flexibly meeting throughput requirements for downlink communication and throughput requirements for uplink communication by changing a ratio between downlink communication subframes and uplink communication subframes. For example, FIG. 2 illustrates UL-DL configurations (Config 0 to 6) having different ratios between downlink communication subframes and uplink communication subframes. In FIG. 2, a downlink communication subframe (or DL subframe) is represented by “D,” an uplink communication subframe (or UL subframe) is represented by “U” and a special subframe is represented by “S.” The special subframe is a subframe at the time of switching from a downlink communication subframe to an uplink communication subframe. In the special subframe, downlink data communication may be performed as in the case of downlink communication subframes. In each UL-DL configuration shown in FIG. 2, subframes corresponding to 2 frames (20 subframes) are expressed in two stages: subframes “D” and “S” in the upper row shown in a hatch pattern used for downlink communication and subframes “U” in the lower row shown in a hatch pattern used for uplink communication.
In FIG. 2, a solid line connecting a downlink communication subframe or special subframe with an uplink communication subframe connects a downlink communication subframe or special subframe for indicating downlink data to a terminal with an uplink communication subframe for reporting an error detection result (e.g., ACK/NACK) for the above-described downlink data to the base station. For example, in Config 0 of FIG. 2, when downlink data is indicated to the terminal in a downlink communication subframe of #0, the error detection result corresponding to the downlink data is reported to the base station in an uplink communication subframe of #4. The error detection result corresponding to the downlink data is reported in an uplink communication subframe located at or after the fourth subframe position from the downlink communication subframe to which the downlink data is assigned.
Furthermore, in FIG. 2, a broken line connecting a downlink communication subframe or special subframe with an uplink communication subframe indicates which downlink communication subframe or special subframe to use to indicate uplink assignment control information (UL grant) corresponding to uplink data indicated by the terminal in the uplink communication subframe. For one uplink communication subframe, there is always one downlink communication subframe, and uplink assignment control information indicating the assignment of uplink data is indicated in a downlink communication subframe that is located at or before the fourth subframe position from an uplink communication subframe to which the uplink data is assigned and that results in a closest subframe difference.
Note that in Config 0, uplink assignment control information indicating the assignment of the uplink data corresponding to uplink data is associated so as to be indicated in downlink communication subframes, 6, 4, 6 and 4 subframes before uplink data transmission in subframes #2, #4, #7 and #9, respectively. Alternatively, it is also possible to switch the association so that indication is performed in downlink communication subframes at seventh subframes before uplink data transmission in subframes #2, #3, #7 and #8 respectively based on a 2-bit uplink communication index (UL index) included in uplink assignment control information indicating the assignment of uplink data. Moreover, not only switching between the two but also association may be performed so that indication is performed in downlink communication subframes 6, 4, 6 and 4 subframes before uplink data transmission in subframes #2, #4, #7 and #9 respectively, and at the same time indication may be performed in downlink communication subframes seventh subframes before uplink data transmission in subframes #2, #3, #7 and #8, respectively.
As shown in FIG. 2, in the UL-DL configuration, it is possible to select a ratio between uplink communication subframes and downlink communication subframes (including special subframes) from a range of 3:2 to 1:9. However, since the UL-DL configuration is a configuration common to all the terminals within a cell, switching is generally not performed in actual operations in consideration of interference to other systems or fluctuations from other systems. As the ratio between uplink communication subframes and downlink communication subframes in actual operations, 3:2 (Config 0), 2:3 (Config 1), 1:1 (Config 6) or the like are used so that both uplink communication and downlink communication can be equally performed. It is not a common practice to configure a UL-DL configuration specified for downlink communication such as Config 2 and Config 5 with ratios of 1:4, 1:9 or the like.
In an LTE-A system, as shown in FIG. 3A, a heterogeneous network is configured which includes a large cell (macro cell) covered by a macro base station and a small cell (picocell or femtocell) covered by a pico base station or femto base station. While many terminals are connected to a macro base station, a small number of terminals are connected to a pico base station or femto base station. In a macro base station, since the ratio between uplink communication traffic and downlink communication traffic is smoothed out among many terminals, there is less fluctuation in the ratio between uplink communication traffic and downlink communication traffic in a short time. That is, since many terminals are connected to the macro base station, some terminals may perform uplink communication while other terminals may perform downlink communication for a certain period, and the ratio between uplink communication traffic and downlink communication traffic is less likely to be significantly concentrated on either uplink communication or downlink communication. On the other hand, since a pico base station or femto base station connects only a small number of terminals, the ratio between uplink communication traffic and downlink communication traffic is more likely to be significantly concentrated on either uplink communication or downlink communication.
Thus, in the LTE-A system, studies are being carried out on temporally switching UL-DL configurations in accordance with a fluctuation in the ratio between uplink communication traffic and downlink communication traffic as shown in FIG. 3B.
There are various switching methods such as (1) a method that changes UL-DL configuration settings using SIB, (2) a method that indicates a UL-DL configuration which is different from the SIB-based setting, through RRC signaling and (3) a method that dynamically switches between uplink communication subframes and downlink communication subframes in subframe units. Generally, (1) requires the longest time for switching. In contrast, (3) is switching in subframe units and allows switching with the least amount of delay. It is effective to dynamically perform switching in subframe units as in (3) in order to more quickly respond to a fluctuation in the ratio between uplink communication traffic and downlink communication traffic.
FIG. 4A and FIG. 4B show an example of dynamically switching between UL-DL configurations in accordance with a fluctuation in the ratio between uplink communication traffic and downlink communication traffic with reference to NPL 4. In the drawings, an example is shown in which an uplink communication subframe of subframe #8 is switched to a downlink communication subframe in a cell for which Config 1 is configured. When there is a DCI intended for the terminal in a downlink communication subframe (subframe #4) corresponding to uplink communication in the uplink communication subframe of subframe #8 (FIG. 4A), the terminal performs uplink communication in the uplink communication subframe of subframe #8.
On the other hand, when there is no DCI intended for the terminal in a downlink communication subframe (subframe #4) corresponding to uplink communication in the uplink communication subframe of subframe #8 (FIG. 4B), the terminal does not perform uplink communication in the uplink communication subframe of corresponding subframe #8. When none of the terminals performs uplink communication, even if the base station performs downlink communication with a certain terminal, the terminal can perform downlink communication without receiving interference caused by uplink communication from other terminals. Therefore, in an uplink communication subframe for which no uplink communication is scheduled, the terminal performs blind decoding on a PDCCH with an assumption that the subframe is a downlink communication subframe. Accordingly, the base station can use the uplink communication subframe as a downlink communication subframe without giving explicit information to the terminal.